This invention relates to optical fiber cables and, more particularly, to optical fiber spans in which different types of optical fibers are used to achieve desired dispersion and loss characteristics.
Signals transmitted through optical fibers are subject to various distorting and attenuating non-linear effects that limit their practical transmission distance. Typically, optical fibers are employed in fiber optic systems that consist of an optical terminal and a plurality of amplifiers/repeaters connected by optical fibers. The amplifier/repeaters are typically situated at regular intervals along a transmission path, and serve to boost the strength of the signal pulses, thereby overcoming the effects of attenuation. The total length of the transmission path is limited by phase shifts in the pulsed signals resulting from the optical nonlinearity of the optical fibers.
As a result of power-dependency of the nonlinear effects, the peaks of the optical pulses in the signal, where the optical power is largest, are repeatedly phase-shifted relative to the tails of the pulses, where power is low. These are Kerr-effect phase shifts. For an optical signal of a given power, the larger the effective area, the smaller the nonlinear phase shift. Therefore, the use of a fiber having a large effective area allows the launch power to be increased, and a lengthening of the span of optical fiber between amplifiers or repeaters.
Part of the nonlinear phase shift can be compensated by chromatic dispersion. However, where partially overlapping pulses undergo nonlinear phase shifts, different phase shifts are induced in the interacting pulses, which cannot be compensated by the same chromatic dispersion.
Another important characteristic of an optical fiber that affects signal transmission is its dispersion. Fiber dispersion causes phase changes in the frequency components of the signal transmitted through the fiber and alters the temporal distribution of the frequency spectrum, and therefore introduces distortion. High dispersion, fiber is, however, more resistant to WDM crosstalk effects, which arises from four wave mixing and cross phase modulation interactions with co-propagating signals at different wavelengths. These interactions are reduced by high local dispersion.
The attenuation provided by a particular fiber design also needs to be considered, as the use of fiber with higher attenuation requires higher power signals to be used.
Various fiber designs exist to provide desired dispersion or loss characteristics. For example, dispersion-shifted fibers (DSF), exhibit zero-dispersion near certain convenient operating wavelengths, for example, near 1550 nm. However, these fibers typically have moderately small effective area and a slightly higher attenuation than standard NSF) fiber. Although operation over long distances is possible in single channel operation, in WDM systems, nonlinear cross talk limits the channel spacing or launch power. Another type of commercially available fiber, known as non-zero dispersion shifted fiber (NZDSF), also often has a small effective area and exhibits a low to moderate dispersion over the transmission window. Other commercially available optical fibers, such as conventional single-mode (SMF) fibers, have large effective areas but exhibit high dispersions near 1550 nm.
It has been recognised that the properties of large effective area and near zero dispersion close to a particular convenient operating wavelength range would be desirable, but these two properties are generally not both found in a single commercially available optical fiber.
Hybrid fiber spans have been proposed, principally with the aim of reducing the total dispersion. One known xe2x80x9cdispersion managedxe2x80x9d optical fiber system comprises an optical fiber span which comprises alternating sections of positive dispersion and negative dispersion characteristics, wherein the total dispersion of the system is close to zero. These fibers provide improved resistance to non-linear crosstalk in WDM systems as a result of the locally high dispersion values. Thus, dispersion managed fibers combine the single channel advantages of low average dispersion with the WDM improvements resulting from the use of locally high dispersion fiber. However, the fibers used must be chosen to have specific dispersion values, and these fibers typically have small effective areas. In particular, the negative dispersion fiber usually requires a small core size and high refractive index difference, giving large scattering losses. As a result, these fiber spans have relatively high loss, for example 0.1 dB/km more than a conventional fiber span of non-dispersion shifted fiber. For a span of 80 km, this represents 8 dB additional loss. Furthermore, a large number of fiber splices may be required within a span which increase the cost and attenuation.
U.S. Pat. No. 5,191,631 describes a hybrid optical fiber comprising a first optical fiber coupled to a second fiber, where the first optical fiber has an effective area substantially larger and a dispersion characteristic substantially lower at a predetermined operating wavelength range than the corresponding properties of the second optical fiber. The first optical fiber with the larger effective area and positive dispersion characteristics is placed after the terminal or repeater and before the second optical fiber with the smaller effective area and negative dispersion characteristics. This requires only two separate fiber types. This arrangement is described as reducing non-linear effects, because the large effective area fiber is located in the high power part of the fiber span, and the large effective area thereby acts to reduce the optical power density.
Known hybrid fiber spans, including that disclosed in U.S. Pat. No. 5,191,631 have been designed to achieve near zero dispersion for the full span. The invention is based on the recognition that the majority of the deleterious effects of non-linearity down a span occurs during the first part of the span, and that dispersion arising in a linear part of the fiber span can be corrected. This approach enables the loss of the fiber span to be reduced.
According to the invention, there is provided a hybrid optical fiber, comprising:
a first fiber section comprising a fiber or a concatenation of fibers having first dispersion characteristics at a predetermined operating wavelength;
a second fiber section coupled to the first fiber section to form the hybrid fiber, wherein the first fiber dispersion characteristics are selected to maintain the signal dispersion within desired limits, and the second fiber is optical fiber having lower loss than the first fiber section.
The invention provides dispersion compensation in a first section of the fiber span, because this is where the signal intensity is highest, and therefore the region of the fiber span where these non-linearities have greatest effect. In the second section of the fiber span, the non-linear effects can be ignored, so that linear dispersion arises in the second fiber section, which can therefore be optimised for low loss rather than for dispersion compensation. Fibers can be selected with lower loss when low dispersion in the operating wavelength range is not a requirement. This linear dispersion can be corrected at the end of the fiber span, for example using a further fiber section of opposite dispersion sign, or using Bragg gratings, or other means. The use of unconstrained dispersion fiber for the (longer) second section enables the loss to be kept to a minimum, whereas low non-linear penalties are obtained in the high power region close to the amplifier.
The fibers preferably have largest possible effective area. However, if dispersion is managed in the first fiber section using negative dispersion fiber, the effective area in the first section may be lower than the effective area of the second fiber. The dispersion characteristics in the first section are used to control the degree of pulse broadening and the extent to which neighbouring pulses partially overlap.
The transition between fiber sections may lie near to the point at which the sum of the signal impairments due to fiber nonlinearity and the signal impairment due to fiber loss for the first dispersion managed fiber section is equal to the sum of the signal impairments due to fiber nonlinearity and the signal impairment due to fiber loss for the second fiber section. The transition between the first and second fiber sections is thus at the location at which it is first beneficial to change from the first fiber type to the second fiber type. This is therefore the optimum location for the change.
The length of the first section may be selected such that the signal intensity reduces by at least 3 dB in the first section, so that non-linear effects have very little influence in the second section. The length of the first section may be selected such that the signal intensity reduces by around 6 dB in the first section.
The first fiber section may comprise a fiber with a negative dispersion characteristic at the operating wavelength, or else a first fiber length with a positive dispersion characteristic at the operating wavelength and a second fiber length with a negative dispersion characteristic at the operating wavelength.
The second fiber section preferably comprises non-dispersion shifted fiber (NDSF), having positive dispersion at the operating wavelength, for example greater than 3 ps/nm/km. This type of conventional fiber is available with very low loss. Alternatively, the second fiber could be a pure silica core fiber (PSCF) which can be designed with an effective area up to 120 xcexcm2 with low loss.
The fiber is preferably used in an optical transmission system comprising a source for launching signals into the fiber, and an amplifier for receiving signals from the fiber. The amplifier may then comprises dispersion compensation elements for correcting the dispersion arising in the second fiber section.
The invention also provides methods for reducing the loss in a dispersion managed fiber.